Research targets cells to reduce lignin, boost fermentable sugar

By Susanne Retka Schill | December 28, 2012

Researchers at the Joint BioEnergy Institute are exploring new ways to meet the challenge of recalcitrant cellulosic feedstocks and lower the cost of biomass-based biofuels. The group recently published two papers detailing their work to genetically modify xylan and lignin, resulting in improved conversion characteristics in the model plants used in the research.

Lignin has long been considered the tough nut to crack in the enzymatic conversion of biomass to cellulosic ethanol. Reducing lignin content in plants was initially seen as the solution for lignin’s role in protecting the cellulose and hemicellulose in plant leaves and stems. While researchers were successful in reducing lignin, it came at a cost, Henrik Scheller, vice president for feedstocks division with the JBEI. “Plants have lignin for a reason. Plants tend to be dwarf or fall over if you breed or find mutants that don’t have much lignin. We needed a new approach.”

Xylan was also targeted by the JBEI researchers, to decrease the proportion of xylose in the model plant, while increasing the glucose. Normal yeast readily ferments glucose, Scheller said. And, while researchers are developing xylose-fermenting yeast variants, “they don’t do it very well.” In some cases, cellulosic ethanol developers have focused their pretreatment and conversion efforts on cellulose, discarding the xylose-containing, hemicellulose portion, he added, “but then you throw away half the biomass.”

The new approach started with mutant plants with decreased lignin or xylan, then sought to limit that expression to specific cell types. “Plants that have decreased lignin or xylan, have collapsed vessels,” Scheller explained. The researchers attribute the poor growth characteristics to restricted uptake of water and nutrients from the roots by the collapsed vessels. “We got the idea to try to remove lignin or xylan from all the other cell types, but not from the vessels,” Scheller said. Then, stacking yet another trait, the researchers sought to increase the thickness of the fiber cell walls, while avoiding any thickening of vessel walls that would impede growth.

“In both the xylan and lignin we managed to restore growth, so the plants look normal, but they still have a significant total reduction in lignin or xylan,” Scheller said. “If we take this plant material and do saccharificaiton, we get a lot more sugar out of it in both cases.” Stacking the traits produced about 2.5 times more sugar in the modified model plants, Arabidosis, than in the wild types used as controls in the experiments. Mechanical stress tests confirmed that the plant stems had not been weakened in the process.

Just how that increase in sugar yield will play out in a real-time cellulosic ethanol process is not yet clear, he added. Increased sugar yields may be one result, or more likely, far fewer enzymes may be required for similar yields from unmodified biomass. “To look at the economy in the sugar yield, have to look at the whole process,” he said. The JBEI has developed an economic model, based on corn stover conversion. In the xylan research, the researchers applied the conversion improvements quantified in their studies to the virtual plant in the economic model to project a 10 to 15 percent lower break-even cost of production.

The researchers have begun work on applying the new genetic modifications to switchgrass varieties being developed in other JBEI research, Scheller said. He expects that about a year from now, they will be ready to publish the results of that work and, perhaps in two or three years, modified switchgrass varieties will be ready to move from the controlled greenhouse environment to field trials. Any field trials of the genetically modified switchgrass will need to be approved, he added. While the switchgrass work is already underway, team also hopes to see whether the traits can be successfully expressed in poplar.

The Joint BioEnergy Institute is a U.S. DOE research center based in the San Francisco area that draws on the expertise and capabilities of four national laboratories, two University of California research universities and a scientific foundation. The multi-institutional partnership is led by Lawrence Berkeley National and includes the Sandia National Laboratories, the University of California campuses of Berkeley and Davis, the Carnegie Institution for Science, the Lawrence Livermore National Laboratory and, starting in October 2012, the Pacific Northwest National Laboratory.